Audio forensics is the close examination of audio files for legal purposes. Audio forensic experts examine the recording in a forensics lab to determine its authenticity. The examination typically involves checking integrity, improving speech clarity, transcribing dialogue, comparing voices, and reconstructing event timelines.
HOW AUDIO EVIDENCE IS COLLECTED
Audio evidence collection must be performed per industry best practices. The Scientific Working Group on Digital Evidence (SWGDE) outlines acceptable methods for acquiring audio files for forensic analysis:
Chain of custody: The chain of custody must be examined to answer questions about the source of the audio evidence and how it was handled from when it was created to when it was delivered to the courtroom. Law enforcement may keep a chain of custody log, which strengthens the file’s authenticity. An audio forensic expert must document file retrieval with video recordings and images to maintain consistency and accuracy in the chain of custody.
Request the original: A forensic audio expert may request the original copy of the audio file or the earliest version. If the original file is digital, it can be copied exactly for authenticity. If the original file is on analog media, it cannot be copied exactly.
Acceptable retrieval: An expert must retrieve an audio file in a way that preserves it as close to its original condition as possible. These retrieval methods include a forensic image of the storage medium or original file, the original recorder or a file transfer from the original storage medium.
FORENSIC AUDIO AUTHENTICATION AND ANALYSIS
After the evidence has been collected, the audio forensics expert will inspect the recording to determine its authenticity. In this digital age of advanced software editing, it is easy for an audio file to be tampered with, making it inauthentic. The expert will examine the recording using SWGDE best practices to determine if an audio file has integrity.
Audio files can also be enhanced to remove unwanted background noise so dialogue can be heard more clearly. Various filters are applied to the recording to achieve this, such as:
Equalizers cut or boost specific bands of frequency. The frequency bands that contain the most speech content — 600-3,000 hertz — can be isolated or amplified to make speech more intelligible. If there are loud background noises, a spectrum analyzer can be used to identify their frequency and reduce the noise with the equalizer.
If the speech or sounds in the recording are faint, the signal can be compressed to reduce the dynamic range and boost the softer sounds.
We have a licensed arsenal of forensic software tools for specific applications like De-Reverb, De-Wind De-Hum and De-Crackle.
HOW FORENSIC AUDIOLOGY IS USED IN COURT
Audio files that have been examined and clarified by an audio forensic expert can be used in court as evidence. The audio forensic scientist may serve as an expert witness and provide reports and testimony on the results of their investigation. The information they provide includes everything from their methodology to unbiased opinions on the results obtained.